西藏次玛班硕地区由秋米斑岩体锆石U-Pb年龄、地球化学特征
Zircon U-Pb Ages and Geochemical Characteristics of Youqiumi Porphyry Pluton in Cimabanshuo Area,Tibet

西藏次玛班硕地区含矿斑岩体具备斑岩型铜矿成矿有利地质条件，结合本地区野外工作的新认识和新发现，展开该地区成岩成矿地质背景及成因等问题研究.对次玛班硕地区由秋米斑岩体开展了详细的锆石U-Pb测年及岩石地球化学研究，结果表明：花岗闪长斑岩LA-ICP-MS锆石U-Pb年龄为15.2±0.8 Ma~15.2±0.7 Ma，即形成时代为中新世.花岗闪长斑岩具有高硅(SiO2=65.08%～66.85%)、高钾(K2O=3.85%～4.58%)、富碱(K2O+Na2O=7.87%～8.90%)、贫镁(MgO= 1.51%～ 1.84%)、准铝质(A/CNK=0.88～0.95)地球化学特征.稀土元素配分模式右倾，具有弱的铕负异常，(La/Yb)N= 36.60～ 47.43，富集强不相容元素(Rb、Th、U、K)和亏损高场强元素(Nb、Ta、Ce、P、Ti)；特殊地，岩石具有典型埃达克岩地球化学特征，即：高Sr(674×10-6 ～876×10-6)、低Yb(0.560×10-6～0.757×10-6)、低Y(7.97×10-6～9.98×10-6)、高Sr/Y比(73.84～109.98).全岩Sr-Nd同位素组成中(87Sr/86Sr)i=0.707878、εNd(t)=-8.26，Nd二阶段模式年龄t2DM= 1 503 Ma，说明其岩浆源于基性下地壳部分熔融.结合区域的火成岩，认为岩石形成于印度大陆与亚洲大陆后碰撞伸展构造背景.
The ore-bearing porphyry body has favorable geological conditions for porphyry copper mineralization in Cimabanshuo area, Tibet. Therefore, it is of great significance to study the geological background and genesis of diagenesis and mineralization in this area. In this paper,combined with their new understanding and new discoveries in the field works, the authors carried out a detailed research of zircon U-Pb dating and petrogeochemistry, for Youqiumi pluton in Cimabanshuo area.The results show that the granodioriteporyphy yields an LA-ICP-MS zircon U-Pb age of 15.2±0.8 Ma to 15.2±0.7 Ma, namely the formation age of Miocene. Chemical analyses show that the granodioriteporyphy has a characteristic of high SiO2 (65.08%-66.85% ), high K2O (3.85%-4.58%), rich K2O+Na2O (7.87%-8.90%), depleted MgO (1.51%-1.84%), meta-aluminum (A/CNK=0.88-0.95). Chondritenormalized REE distribution pattern diagrams display rightincline and weakly negative Eu anomalies,differentiated obviously (La/Yb)N=36.60-47.43. Enriched strongly in compatible elements such as Rb, Th, U, K and depleted in high field strength elements (HFSEs) such as Nb, Ta, Ce, P, Ti. Specifically, Youqiumi granitic pluton shows typical adakitic signatures: high Sr (674×10-6 to 876×10-6) and low Yb (0.560×10-6 to 0.757×10-6) and low Y (7.97×10-6 to 9.98×10-6), contents with high Sr/Y ratios (73.84-109.98). However, the wholerock Sr, Nd isotopic compositions of granodioriteporyphy have initial 87 Sr/ 86Sr ratios and εNd(t) values are 0.707 878 and -8.26 respectively, with ancient Nd twophase model ages of 1 503 Ma，which indicate that the magma sources may have contributed to partial melting of previous juvenile lower crust.Combined with igneous rocks in the studied area, it is proposed that Youqiumi granitic pluton was formed in postcollisional extension setting of IndiaAsia contional collision orogenic belt